Thermodynamic properties for the solution of hydrogen in palladium and its alloys are experimentally determined under conditions of constant pressure (corresponding to a free surface solid). In this research these data are converted to conditions of constant alloy volume. For palladium/hydrogen it is predicted that the phase change, α→β, which is so characteristic of this system, no longer appears under conditions of constant volume. This is indicative of the fact that long range dipole-dipole elastic interactions predominate in this system. A different sequence of behaviour of ΔµxsH→0 values with atom fraction of substituted metal is predicted for alloys of the type palladium + group IB for constant volume conditions as compared with conditions of constant pressure.